I think that's the crux of the issue right there. Most people don't like to share, especially if they've never thought of the concept of sharing before. When my boyfriend and I first got together around 5 years ago, I suggested the idea to him. He came from a very strict religious background (in fact he was still a virgin when we met), so it wasn't something he was ready to jump into. After we'd been together a year though, he brought it up again and said he was curious and wanted to give it a try. We had a bit of a rocky start, but overall it's been going well for us.
That is cool it worked for you. I just never got really into the relationship either time I tried. For my friend though has worked great. She has a husband and a bf. The guys are straight, one is questionably bi imo, and they have three sons. Not everyone's thing but it does work.